Electric compressor

ABSTRACT

An electric compressor includes a housing; a rotary shaft; an impeller connected to at least a first end portion of the rotary shaft in an axial direction of the rotary shaft, of the first end portion and a second end portion of the rotary shaft in the axial direction; and a pair of air bearings supporting the rotary shaft such that the rotary shaft is rotatable relative to the housing. A load on the first end portion is larger than a load on the second end portion. The pair of air bearings includes a first air bearing, and a second air bearing supporting the rotary shaft at a position closer to the second end portion of the rotary shaft than the first air bearing is. A load carrying capacity of the first air bearing is larger than a load carrying capacity of the second bearing.

TECHNICAL FIELD

The present invention relates to an electric compressor.

BACKGROUND ART

Patent Literature 1 mentions an electric compressor that includes ahousing having therein a space, a rotary shaft accommodated in thehousing, an impeller connected to one end of the rotary shaft in anaxial direction of the rotary shaft, and a pair of air bearingssupporting the rotary shaft such that the rotary shaft is rotatablerelative to the housing. The rotation of the rotary shaft forms an airfilm between the outer peripheral surface of the rotary shaft and theair bearings, thereby causing the rotary shaft to float off the airbearings. This allows the air bearings to support the rotary shaftwithout coming into contact with the rotary shaft.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Publication No.2011-188612

SUMMARY OF INVENTION Technical Problem

In the electric compressor mentioned in Patent Literature 1, althoughone end of the rotary shaft in the axial direction of the rotary shaftis connected to the impeller, the other end of the rotary shaft is notconnected to an impeller.

Accordingly, compression is not performed near the other end of therotary shaft although compression is performed by the impeller near theone end of the rotary shaft, so that a load applied by the rotation ofthe rotary shaft on the one end of the rotary shaft is different from aload applied by the rotation of the rotary shaft on the other end of therotary shaft. Some of electric compressor may include impellersrespectively connected to opposite ends of the rotary shaft. However,the impellers of such electric compressors may provide differentcompression capacities between the one end and the other end of therotary shaft due to a difference in size between the impellers, so thatthe load applied by the rotation of the rotary shaft on the one end ofthe rotary shaft may be different from the load applied by the rotationof the rotary shaft on the other end of the rotary shaft.

If different loads, i.e., a large load and a small load, are applied bythe rotation of the rotary shaft on the opposite ends of the rotaryshaft, a necessary load carrying capacity is different between the airbearings respectively disposed on the opposite ends of the rotary shaft.If the air bearings have the same load carrying capacity, this loadcarrying capacity is excessive for one of the air bearings but deficientfor the other of the air bearings. Deficient load carrying capacity ofthe air bearing may deteriorate the air bearing early. Excessive loadcarrying capacity of the air bearing may increase the manufacturing costof the air bearing.

The present invention, which has been made in light of theabove-mentioned problem, is directed to providing an electric compressorthat is capable of preventing an excess or a deficiency of load carryingcapacity of an air bearing relative to a necessary load carryingcapacity of the air bearing.

Solution to Problem

An electric compressor to improve the above-mentioned problemcomprising: a housing having therein a space; a rotary shaftaccommodated in the housing; an impeller connected to at least a firstend portion of the rotary shaft in an axial direction of the rotaryshaft, of the first end portion and a second end portion of the rotaryshaft in the axial direction; and a pair of air bearings supporting therotary shaft such that the rotary shaft is rotatable relative to thehousing, wherein a load on the first end portion is larger than a loadon the second end portion, the pair of air bearings includes a first airbearing, and a second air bearing supporting the rotary shaft at aposition closer to the second end portion of the rotary shaft than thefirst air bearing is, and a load carrying capacity of the first airbearing is larger than a load carrying capacity of the second bearing.

When the rotation of the rotary shaft applies a larger load on the firstend portion than on the second end portion because the impeller isconnected only to the first end portion, the rotary shaft applies alarger load on the first air bearing than on the second air bearing.Also, when the rotation of the rotary shaft applies a larger load on thefirst end portion than on the second end portion although the first endportion and the second end portion are respectively connected toimpellers, the rotary shaft applies a larger load on the first airbearing than on the second air bearing.

Accordingly, the first air bearing needs a relatively large loadcarrying capacity, and the second air bearing needs a relatively smallload carrying capacity. This load carrying capacity is a maximum loadthat each air bearing can receive without a deformation and aperformance deterioration.

According to this configuration, the load carrying capacity of the firstair bearing is larger than the load carrying capacity of the secondbearing. This prevents a deficiency of the load carrying capacity of thefirst air bearing and an excess of the load carrying capacity of thesecond air bearing. This therefore prevents an excess or a deficiency ofthe load carrying capacity of each air bearing relative to a necessaryload carrying capacity of the air bearing.

In the electric compressor, a length of the first air bearing may bepreferably greater than a length of the second air bearing in the axialdirection, so that the load carrying capacity of the first air bearingmay be preferably larger than the load carrying capacity of the secondair bearing.

According to this configuration, the length of the first air bearing isgreater than the length of the second air bearing in the axialdirection, so that a supporting surface of the first air bearing forsupporting the rotary shaft is larger than a supporting surface of thesecond air bearing for supporting the rotary shaft. This allows the loadcarrying capacity of the first air bearing to be larger than the loadcarrying capacity of the second air bearing just by making a differencein length in the axial direction between the first air bearing and thesecond air bearing without changing the shapes of the first air bearingand the second air bearing. This therefore easily prevents an excess ora deficiency of the load carrying capacity of each air bearing relativeto a necessary load capacity of the air bearings.

In the electric compressor, the first air bearing and the second airbearing may have different shapes so that the load carrying capacity ofthe first air bearing is larger than the load carrying capacity of thesecond air bearing.

ADVANTAGEOUS EFFECTS OF INVENTION

This disclosure prevents an excess or a deficiency of a load carryingcapacity of each air bearing relative to a necessary load carryingcapacity of each air bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an electric compressor.

FIG. 2 is an exploded perspective view of a rotary shaft and a first airbearing.

FIG. 3 is a sectional view of an air bearing mounted on the rotaryshaft.

FIG. 4 is an enlarged sectional view of the air bearing mounted on therotary shaft.

FIG. 5 is a schematic view explaining the length of the first airbearing and the length of a second air bearing.

FIG. 6 is a sectional view of an air bearing mounted on a rotary shaftaccording to an example.

FIG. 7 is a sectional view of an air bearing mounted on a rotary shaftaccording to another example.

FIG. 8 is a sectional view of an air bearing mounted on a rotary shaftaccording to another example.

FIG. 9 is an exploded perspective view of a rotary shaft and a first airbearing according to another example.

DESCRIPTION OF EMBODIMENTS

The following will describe an embodiment of an electric compressor withreference to accompanying FIGS. 1 to 5 .

As illustrated in FIG. 1 , an electric compressor 10 includes a housing11 having a cylindrical shape and therein a space, and an electric motor20 accommodated in the housing 11. The housing 11 includes a firsthousing member 12 having a plate-like shape and a second housing member13 having a bottomed-cylindrical shape and connected to the firsthousing member 12. The first housing member 12 and the second housingmember 13 are each made of a metallic material, such as aluminum. Thesecond housing member 13 has a bottom wall 13 a having a plate-likeshape and a peripheral wall 13 b having a cylindrical shape andextending from an outer peripheral portion of the bottom wall 13 a. Thefirst housing member 12 is connected to the second housing member 13with an opening of the peripheral wall 13 b distant from the bottom wall13 a closed by the second housing member 13.

The first housing member 12 has a housing hole 12 c that is formedthrough the first housing member 12 in the thickness direction of thefirst housing member 12. The housing hole 12 c is a circular hole. Thesecond housing member 13 has a cylindrical boss 13 c protruding from theinner surface of the bottom wall 13 a. The axis of the housing hole 12 cis coaxial with the axis of the boss 13 c.

The electric motor 20 includes a stator 21 and a rotor 22. The stator 21includes a cylindrical stator core 21 a that is fixed to the innerperipheral surface of the peripheral wall 13 b of the second housingmember 13, and a coil 21 b that is wound around the stator core 21 a.The rotor 22 is rotatably disposed radially inside the stator 21 in thehousing 11.

The rotor 22 includes a cylindrical member 23, a permanent magnet 24 asa magnetic body, and a rotary shaft 25. The cylindrical member 23 has acircular cylindrical shape. The axis of the cylindrical member 23corresponds to the axes of the housing hole 12 c and the boss 13 c. Inthis embodiment, a direction along the axis of the cylindrical member 23is called an axial direction. A direction along the radius of thecylindrical member 23 is called a radial direction. The cylindricalmember 23 has a first opening 23 a and a second opening 23 brespectively at opposite ends of the cylindrical member 23 in the axialdirection. The cylindrical member 23 is made of a metallic material,such as titanium.

The permanent magnet 24 has a solid column shape and is magnetized inthe radial direction. The permanent magnet 24 is press-fitted in theinner peripheral surface of the cylindrical member 23 so as to be fixedinto the cylindrical member 23. The axis of the permanent magnet 24corresponds to the axis of the cylindrical member 23. The length of thepermanent magnet 24 is shorter than that of the cylindrical member 23 inthe axial direction.

The rotary shaft 25 has a column-shaped first shaft portion 26 and acolumn-shaped second shaft portion 27 respectively located oppositesides in the axial direction with respect to the permanent magnet 24.The first shaft portion 26 and the column-shaped second shaft portion 27are made of metal, for example. The first shaft portion 26 has a firstsmall-diameter shaft portion 26 a, and a first large-diameter shaftportion 26 b having a diameter larger than that of the firstsmall-diameter shaft portion 26 a and aligned with the firstsmall-diameter shaft portion 26 a in the axial direction. The axis ofthe first small-diameter shaft portion 26 a and the axis of the firstlarge-diameter shaft portion 26 b extend along the axial direction. Thesecond shaft portion 27 has a second small-diameter shaft portion 27 a,and a second large-diameter shaft portion 27 b having a diameter largerthan that of the second small-diameter shaft portion 27 a and alignedwith the second small-diameter shaft portion 27 a in the axialdirection. The axis of the second small-diameter shaft portion 27 a andthe axis of the second large-diameter shaft portion 27 b extend alongthe axial direction. The first small-diameter shaft portion 26 a and thesecond small-diameter shaft portion 27 a have the same diameter. Thefirst large-diameter shaft portion 26 b and the second large-diametershaft portion 27 b have the same diameter.

The first large-diameter shaft portion 26 b is located in the housinghole 12 c of the first housing member 12. The second large-diametershaft portion 27 b is located in the boss 13 c. The first small-diametershaft portion 26 a is inserted through the first opening 23 a of thecylindrical member 23 and fixed to the cylindrical member 23 so as toclose the first opening 23 a. The second small-diameter shaft portion 27a is inserted through the second opening 23 b of the cylindrical member23 and fixed to the cylindrical member 23 so as to close the secondopening 23 b. This configuration allows the first shaft portion 26 andthe second shaft portion 27 to be rotatable together with thecylindrical member 23 and the permanent magnet 24.

The axis of each of the first shaft portion 26 and the second shaftportion 27, i.e., the axis of the rotary shaft 25, corresponds to thecylindrical member 23. The axis of the rotary shaft 25 is illustrated bythe axis L.

One end of the opposite ends of the first large-diameter shaft portion26 b in the axial direction is connected to the first small-diametershaft portion 26 a, and the other end of the opposite ends serves as afirst end portion 25 a of the rotary shaft 25. One end of the oppositeends of the second large-diameter shaft portion 27 b in the axialdirection is connected to the second small-diameter shaft portion 27 a,and the other end of the opposite ends serves as a second end portion 25b of the rotary shaft 25. In this embodiment, the first end portion 25 aof the rotary shaft 25 is connected to an impeller 32.

The impeller 32 includes an impeller shaft 32 a extending in the axialdirection, a hub 32 b fixed to an outer peripheral surface of theimpeller shaft 32 a and configured to rotate together with the impellershaft 32 a, and a plurality of vanes 32 c arranged in thecircumferential direction of the hub 32. The impeller shaft 32 a extendsfrom the first end portion 25 a of the rotary shaft 25 in the axialdirection so that the impeller shaft 32 a protrudes outside the housing11. The hub 32 b has an approximately conical shape and an outerdiameter of the hub 32 b expands as the hub 32 b extends from one sideto the other side in the axial direction. The vanes 32 c are disposed onthe outer surface of the hub 32 b and equally spaced from each other inthe circumferential direction of the hub 32 b.

The first housing member 12 is connected to a compressor housing 31 thathas a cylindrical shape and has an inlet 31 a. The compressor housing 31has the inlet 31 a at one end thereof in the axial direction. The inlet31 a extends in the axial direction. The other end of the compressorhousing 31 has an opening that is closed by the first housing member 12.The compressor housing 31 has therein an impeller chamber 33 in whichthe impeller 32 is accommodated. The impeller chamber 33 is communicatedwith the inlet 31 a. The impeller shaft 32 a extends in the axialdirection in the impeller chamber 33.

The compressor housing 31 has a discharge chamber 34 in which aircompressed by the impeller 32 is discharged, and a diffuser passage 35through which the impeller chamber 33 is communicated with thedischarged chamber 34. The diffuser passage 35 is located outward of theimpeller chamber 33 in the radial direction of the impeller shaft 32 aand formed into a ring shape surrounding the impeller chamber 33. Thedischarge chamber 34 is located outward of the diffuser passage 35 inthe radial direction of the impeller shaft 32 a and formed into a ringshape.

In the electric compressor 10, the rotor 22 including the rotary shaft25 is rotated by energization of the coil 21 b. The rotation of therotary shaft 25 rotates the impeller 32 so as to compress the air drawnfrom the inlet 31 a into the impeller chamber 33. The air compressed bythe impeller 32 is further compressed via the diffuser passage 35 and isdischarged to the discharge chamber 34. The air in the discharge chamber34 is discharged outside the compressor housing 31 from an outlet (notillustrated) of the compressor housing 31.

In the electric compressor 10 according to the embodiment, although thefirst end portion 25 a of the rotary shaft 25 is connected to theimpeller 32, the second end portion 25 b of the rotary shaft 25 is notconnected to an impeller. That is, in the electric compressor 10according to the embodiment, although compression is performed by theimpeller 32 near the first end portion 25 a of the rotary shaft 25,compression is not performed near the second end portion 25 b of therotary shaft 25. Accordingly, in the electric compressor 10 according tothe embodiment, a load applied by the rotation of the rotary shaft 25 onthe first end portion 25 a is larger than that on the second end portion25 b.

The rotary shaft 25 is rotatably supported by a pair of air bearings 40relative to the housing 11. The pair of air bearings 40 includes a firstair bearing 41 supporting the first shaft portion 26 and a second airbearing 42 supporting the second shaft portion 27. That is, the secondair bearing 42 supports the rotary shaft 25 at a position closer to thesecond end portion 25 b of the rotary shaft 25 than the first airbearing 41 is.

The first air bearing 41 and the second air bearing 42 have acylindrical shape. The axis of the first air bearing 41 and the axis ofthe second air bearing 42 correspond to the axis L of the rotary shaft25. The first air bearing 41 is disposed between the inner peripheralsurface of the housing hole 12 c of the first housing member 12 and theouter peripheral surface of the first large-diameter shaft portion 26 b.The second air bearing 42 is disposed between the inner peripheralsurface of the boss 13 c of the second housing member 13 and the outerperipheral surface of the second large-diameter shaft portion 27 b. Therotary shaft 25 is supported by the housing 11 via the first air bearing41 and the second air bearing 42 such that the rotary shaft 25 isrotatable relative to the housing 11.

The rotary shaft 25 is supported by the first air bearing 41 and thesecond air bearing 42 with the rotary shaft 25 in contact with the firstair bearing 41 and the second air bearing 42 until the rotational speedof the rotary shaft 25 reaches a floating rotational speed at which therotary shaft 25 floats off the first air bearing 41 and the second airbearing 42. When the rotational speed of the rotary shaft 25 reaches thefloating rotational speed, a dynamic pressure is generated between thefirst air bearing 41 and the first shaft portion 26 and between thesecond air bearing 42 and the second shaft portion 27. The dynamicpressure allows the rotary shaft 25 to float off the first air bearing41 and the second air bearing 42, so that the rotary shaft 25 issupported by the first air bearing 41 and the second air bearing 42without coming into contact with the first air bearing 41 and the secondair bearing 42. The first air bearing 41 and the second air bearing 42are air dynamic bearings that support the rotary shaft 25 in the radialdirection.

Next, the following will describe the air bearings 40 in more detail.The first air bearing 41 and the second air bearing 42 have the samebase configuration. Accordingly, the following description will focus onthe configuration of the first air bearing 41, and will not elaboratethe same components of the second air bearing 42 as that of the firstair bearing 41.

As illustrated in FIGS. 2 and 3 , the first air bearing 41 includes atop foil 45 that has an approximately cylindrical shape and surroundsthe rotary shaft 25 so as to support the rotary shaft 25, and a bumpfoil 50 that has an approximately cylindrical shape and surrounds thetop foil 45. The outer peripheral surface of the bump foil 50 issupported by a bearing housing 55 that has a cylindrical shape andsurrounds the bump foil 50. The axis of each of the top foil 45, thebump foil 50, and the bearing housing 55 corresponds to the axis L ofthe rotary shaft 25.

The first air bearing 41 has a configuration in which the top foil 45,the bump foil 50, and the bearing housing 55 are disposed between theouter peripheral surface of the first large-diameter shaft portion 26 bof the first shaft portion 26 and the inner peripheral surface of thehousing hole 12 c of the first housing member 12. The second air bearing42 has a configuration in which the top foil 45, the bump foil 50, andthe bearing housing 55 are disposed between the outer peripheral surfaceof the second large-diameter shaft portion 27 b of the second shaftportion 27 and the inner peripheral surface of the boss 13 c of thesecond housing member 13. The rotary shaft 25 rotates in a clockwisedirection indicated by an arrow X in FIG. 3 .

The top foil 45 is formed of a flexible metallic plate, such as a nickelalloy plate, curved into a cylindrical shape. One of the opposite endsof the top foil 45 in a circumferential direction of the top foil 45 isa first fixed end 45 a that is fixed to the bump foil 50. The firstfixed end 45 a extends outwardly in the radial direction of the top foil45. The other of the opposite ends of the top foil 45 is a first freeend 45 b that is not fixed to the bump foil 50. The first free end 45 bis spaced from the first fixed end 45 a in the circumferential directionof the top foil 45. Since the top foil 45 has an approximatelycylindrical shape, the distance between the first fixed end 45 a and thefirst free end 45 b is small.

The bump foil 50 is formed of a flexible metallic plate, such as anickel alloy plate, and extends along the outer peripheral surface ofthe top foil 45. One of the opposite ends of the bump foil 50 in thecircumferential direction of the bump foil 50 is a second fixed end 50 athat is fixed to the inner peripheral surface of the bearing housing 55.The first fixed end 45 a of the top foil 45 is placed on and fixed tothe second fixed end 50 a. That is, the first fixed end 45 a is fixed tothe inner peripheral surface of the bearing housing 55 via the secondfixed end 50 a. The other of the opposite ends of the bump foil 50 is asecond free end 50 b that is not fixed to the bearing housing 55. Thesecond free end 50 b is spaced from the second fixed end 50 a in thecircumferential direction of the bump foil 50. Since the bump foil 50has an approximately cylindrical shape, the distance between the secondfixed end 50 a and the second free end 50 b is small.

As illustrated in FIG. 4 , the bump foil 50 has a plurality ofprojections 51 that project in the radial direction of the bump foil 50.The projections 51 are spaced from each other in the circumferentialdirection of the bump foil 50. Each of the projections 51 issemi-circular in cross-section in a direction perpendicular to the axialdirection. In the bump foil 50, the adjacent projections 51 areconnected to each other by an extending portion 52 that extends in thecircumferential direction of the bump foil 50. The extending portion 52extends along the inner peripheral surface of the bearing housing 55,and each of the projections 51 projects so as to be radially andinwardly spaced from the inner peripheral surface of the bearing housing55. The bump foil 50 is formed into a corrugated shape as a whole.

The extending portion 52 of the bump foil 50 and the top of theprojection 51 are respectively in contact with the inner peripheralsurface of the bearing housing 55 and the outer peripheral surface ofthe top foil 45 when the rotary shaft 25 is not rotated. The top foil 45is elastically and radially outwardly deformed when the rotary shaft 25is rotated, so that air enters a gap between the outer peripheralsurface of the rotary shaft 25 and an inner peripheral surface 45 c ofthe top foil 45 to form an air film. That is, the rotary shaft 25 issupported by the inner peripheral surface 45 c of the top foil 45 viathe air film. The inner peripheral surface 45 c of the top foil 45serves as a supporting surface that supports the rotary shaft 25. Theelastic and radially outward deformation of the top foil 45 along withthe formation of the air film causes the bump foil 50 to be elasticallyand radially outwardly deformed via the projections 51 in contact withthe outer peripheral surface of the top foil 45.

The bump foil 50 has a first thickness T1 in both of the first airbearing 41 and the second air bearing 42. The thickness of the bump foil50 corresponds to the thickness of the metallic plate that forms thebump foil 50. The bump foil 50 of the first air bearing 41 and the bumpfoil 50 of the second air bearing 42 have the same number of theprojections 51 in a predetermined length L3 in the circumferentialdirection of the bump foil 50. In other words, the first air bearing 41and the second air bearing 42 have the same area density of theprojections 51 in their bump foils 50. Each of the projections 51 formsa first angle A1 with the corresponding extending portion 52 in aboundary between the projection 51 and the extending portion 52 in thecircumferential direction of the bump foil 50 in both of the first airbearing 41 and the second air bearing 42. The first angle A1 is greaterthan 0 degrees and less than 90 degrees. In this embodiment, the firstair bearing 41 and the second air bearing 42 have the same thickness ofthe bump foil 50, the same area density of the projections 51, and thesame angle formed by each projection 51 and the corresponding extendingportion 52, so that the first air bearing 41 and the second air bearing42 have the same shape.

The circumferential length of the top foil 45 of each of the first airbearing 41 and the second air bearing 42 is determined so that the wholeof the inner peripheral surface 45 c of the top foil 45 is in contactwith the outer peripheral surface of the rotary shaft 25 when the rotaryshaft 25 is not rotated. The first air bearing 41 and the second airbearing 42 have the same length of the inner peripheral surface 45 c inthe circumferential direction of the top foil 45. Similarly, the firstair bearing 41 and the second air bearing 42 have the same length of thebump foil 50 and the same length of the bearing housing 55 in thecircumferential direction.

As illustrated in FIG. 2 , the top foil 45, the bump foil 50, and thebearing housing 55 of the first air bearing 41 have the same length inthe axial direction. The top foil 45, the bump foil 50, and the bearinghousing 55 of the second air bearing 42 have the same length in theaxial direction. The length of the bearing housing 55 may be slightlygreater than the length of the top foil 45 and the length of the bumpfoil 50 in the axial direction.

As illustrated in FIG. 5 , the top foil 45 of the first air bearing 41has a first length L1 in the axial direction, and the top foil 45 of thesecond air bearing 42 has a second length L2 that is shorter than thefirst length L1. That is, the area of the inner peripheral surface 45 cof the top foil 45 of the first air bearing 41 is larger than that ofthe second air bearing 42. The larger area of the inner peripheralsurface 45 c of the top foil 45 extends the supporting surface, whichsupports the rotary shaft 25 via the air film when the rotary shaft 25is rotated, thereby increasing the load carrying capacity of the airbearings 40. Accordingly, in this embodiment, the load carrying capacityof the first air bearing 41 is larger than the load carrying capacity ofthe second air bearing 42.

Next, the following will explain the operation of the electriccompressor according to the embodiment.

When the rotary shaft 25 is rotated, air enters a gap between the outerperipheral surface of the rotary shaft 25 and the inner peripheralsurface 45 c of the top foil 45 and forms an air film. This causes thetop foil 45 to be elastically and radially outwardly deformed andtherefore the bump foil 50 to be elastically and radially outwardlydeformed via the projections 51 in contact with the outer peripheralsurface of the top foil 45.

The rotation of the rotary shaft 25 applies a larger load on the firstair bearing 41, which supports the rotary shaft 25 at a positionadjacent to the first end portion 25 a to which the impeller 32 isconnected, than that on the second air bearing 42. Accordingly, thefirst air bearing 41 needs a relatively large load carrying capacity,and the second air bearing 42 needs a relatively small load carryingcapacity.

In this embodiment, since the area of the inner peripheral surface 45 cof the top foil 45 of the first air bearing 41 is larger than that ofthe second air bearing 42, the load carrying capacity of the first airbearing 41 is larger than that of the second air bearing 42. This allowsthe respective first air bearing 41 and the second air bearing 42 tohave a load carrying capacity satisfying a necessary load carryingcapacity.

This embodiment provides following advantageous effects.

(1) The load carrying capacity of the first air bearing 41 is largerthan the load carrying capacity of the second air bearing 42. Thisprevents a deficiency of the load carrying capacity of the first airbearing 41 and an excess of the load carrying capacity of the second airbearing 42. This therefore prevents an excess or a deficiency of theload carrying capacity of each of the air bearings 40 relative to anecessary load carrying capacity of each of the air bearings 40.

(2) The length of the first air bearing 41 is greater than that of thesecond air bearing 42 in the axial direction, so that the area of theinner peripheral surface 45 c of the top foil 45 of the first airbearing 41 is larger than that of the second air bearing 42. This allowsthe load carrying capacity of the first air bearing 41 to be larger thanthat of the second air bearing 42 just by a difference in length in theaxial direction between the first air bearing 41 and the second airbearing 42 without changing the shapes of the first air bearing 41 andthe second air bearing 42. This therefore easily prevents an excess or adeficiency of the load carrying capacity of each of the air bearings 40relative to the necessary load carrying capacity of each of the airbearings 40.

This embodiment may be modified as below. The embodiment may be combinedwith the following modification examples within technically consistentrange.

-   -   As illustrated in FIG. 6 , the bump foil 50 of the first air        bearing 41 may have a second thickness T2 that is thicker than        the first thickness T1 of the second air bearing 42. The        difference in thickness between the bump foil 50 of the first        air bearing 41 and the bump foil 50 of the second air bearing 42        makes a difference in shape between the first air bearing 41 and        the second air bearing 42. The thicker bump foil 50 increases        the stiffness of the bump foil 50, thereby increasing the load        carrying capacity in the air bearings 40. In this configuration,        the bump foil 50 of the first air bearing 41 is thicker than        that of the second air bearing 42, so that the load carrying        capacity of the first air bearing 41 is larger than that of the        second air bearing 42.    -   As illustrated in FIG. 7 , the number of the projections 51 of        the bump foil 50 of the first air bearing 41 may be larger than        that of the second air bearing 42 in the predetermined length L3        in the circumferential direction of the bump foil 50. In other        words, the area density of the projections 51 of the bump foil        50 of the first air bearing 41 may be greater than that of the        second air bearing 42. The difference in area density of the        projections 51 between the bump foil 50 of the first air bearing        41 and the bump foil 50 of the second air bearing 42 makes a        difference in shape between the first air bearing 41 and the        second air bearing 42. The greater area density of the        projections 51 of the bump foil 50 increases the stiffness of        the bump foil 50, thereby increasing the load carrying capacity        in the air bearings 40. In this configuration, the area density        of the projections 51 of the bump foil 50 of the first air        bearing 41 is greater than that of the second air bearing 42, so        that the load carrying capacity of the first air bearing 41 is        larger than that of the second air bearing 42.    -   As illustrated in FIG. 8 , each projection 51 of the bump foil        50 may be divided with respect to the circumferential direction        in both of the first air bearing 41 and the second air bearing        42. In this configuration, each projection 51 is formed of a        first projection 51 a and a second projection 51 b adjacent to        each other in the circumferential direction of the bump foil 50.        The first projection 51 a is curved in the rotational direction        of the rotary shaft 25 so as to approach the outer peripheral        surface of the top foil 45 from one end of the extending portion        52. The second projection 51 b is curved in the rotational        direction of the rotary shaft 25 so as to approach one end of        another extending portion 52 from the outer peripheral surface        of the top foil 45. The top of the first projection 51 a is        spaced from the top of the second projection 51 b in the        circumferential direction. Each of the projections 51 is formed        by the first projection 51 a and the second projection 51 b into        approximately semi-circular in cross-section in a direction        perpendicular to the axial direction.    -   In the second air bearing 42 of this modification example, each        of the first projection 51 a and the second projection 51 b        forms the first angle A1 with the corresponding extending        portion 52 in a boundary between the projection 51 and the        extending portion 52 in the circumferential direction of the        bump foil 50. Alternatively, in the first air bearing 41, this        angle may be a second angle A2 that is greater than the first        angle A1 and less than 90 degrees. The first angle A1 and the        second angle A2 are angles when the rotary shaft 25 is not        rotated. The difference in angle formed by each projection 51        and the corresponding extending portion 52 between the first air        bearing 41 and the second air bearing 42 makes a difference in        shape between the first air bearing 41 and the second air        bearing 42. The greater angle formed by the projection 51 and        the extending portion 52 and not exceeding 90 degrees increases        the stiffness of the bump foil 50, thereby increasing the load        carrying capacity in the air bearings 40. In this configuration,        the angle formed by the projection 51 and the extending portion        52 of the first air bearing 41 is greater than that of the        second air bearing 42, so that the load carrying capacity of the        first air bearing 41 is larger than that of the second air        bearing 42. In this configuration, similarly to the embodiment,        each projection 51 may not be divided in both of the first air        bearing 41 and the second air bearing 42. This configuration        allows the angle formed by the projection 51 and the extending        portion 52 of the first air bearing 41 to be greater than that        of the second air bearing 42, so that the load carrying capacity        of the first air bearing 41 is larger than that of the second        air bearing 42.    -   As illustrated in FIG. 9 , the bump foil 50 of the first air        bearing 41 may be divided with respect to the axial direction.        In this configuration, the bump foil 50 is formed of a first        bump foil member 150 a and a second bump foil member 150 b        adjacent to each other in the axial direction. The length of the        first bump foil member 150 a and the length of the second bump        foil member 150 b are half the length of the top foil 45 in the        axial direction. The first bump foil member 150 a and the second        bump foil member 150 b are fixed to the bearing housing 55 with        the first bump foil member 150 a and the second bump foil member        150 b in contact with each other in the axial direction.        Accordingly, in the first air bearing 41, the length of the        whole bump foil 50 is equal to that of the top foil 45 in the        axial direction. In this configuration, similarly to the        embodiment, the bump foil 50 of the second air bearing 42 is not        divided in the axial direction. Accordingly, the top foil 45 of        the first air bearing 41 has the second length L2 that is equal        to the length of the top foil 45 of the second air bearing 42 in        the axial direction. The length of each of the first bump foil        member 150 a and the second bump foil member 150 b is a third        length L4, which is half the length of the second length L2 in        the axial direction.

In this configuration, the bump foil 50 of the first air bearing 41 isdivided into more members than the bump foil 50 of the second airbearing 42 in such a manner, so that the first air bearing 41 and thesecond air bearing 42 have different shapes. More divisions of the bumpfoil 50 in the axial direction allow distribution of the load applied bythe rotary shaft 25 on the bump foil 50, thereby increasing thestiffness of the bump foil 50 and the load carrying capacity in the airbearings 40. In this configuration, the bump foil 50 of the first airbearing 41 is divided into more members than the bump foil 50 of thesecond air bearing 42 in the axial direction, so that the load carryingcapacity of the first air bearing 41 is larger than that of the secondair bearing 42.

-   -   In the modification example as illustrated in FIG. 9 , the bump        foil 50 of the first air bearing 41 may be divided into three or        more members. The bump foil 50 of the second air bearing 42 may        be divided with respect to the axial direction. That is, the        bump foil 50 of the first air bearing 41 and the bump foil 50 of        the second air bearing 42 may be divided into any number of        members in the axial direction as long as the bump foil 50 of        the first air bearing 41 is divided into more members than the        bump foil 50 of the second air bearing 42.    -   The bump foil 50 of the first air bearing 41 and the bump foil        50 of the second air bearing 42 may be made of different        materials. For example, if the bump foil 50 of the first air        bearing 41 may be made of a material with higher Young's modulus        than that of the material of the bump foil 50 of the second air        bearing 42, the load carrying capacity of the first air bearing        41 is larger than that of the second air bearing 42.    -   The top foil 45 and the bump foil 50 may be made of a flexible        metal other than nickel alloy, such as stainless steel.    -   Both of the first end portion 25 a and the second end portion 25        b of the rotary shaft 25 may be respectively connected to        impellers 32. In this configuration, the impeller 32 connected        to the first end portion 25 a may be larger than the impeller 32        connected to the second end portion 25 b, so that the        compression capacity provided by the impeller 32 on the first        end portion 25 a may be larger than that provided by the other        impeller 32 on the second end portion 25 b. That is, a load        applied by the rotation of the rotary shaft 25 on the first end        portion 25 a may be larger than that on the second end portion        25 b. If the load carrying capacity of the first air bearing 41        is set larger than the load carrying capacity of the second air        bearing 42 as explained in the embodiment and the modification        examples for the electric compressor 10 with such a difference        in load, the same advantageous effects as in the above        embodiment can be obtained.

REFERENCE SIGNS LIST

10 electric compressor

11 housing

25 rotary shaft

25 a first end portion

25 b second end portion

32 impeller

40 air bearing

41 first air bearing

42 second air bearing

1. An electric compressor comprising: a housing having therein a space;a rotary shaft accommodated in the housing; an impeller connected to atleast a first end portion of the rotary shaft in an axial direction ofthe rotary shaft, of the first end portion and a second end portion ofthe rotary shaft in the axial direction; and a pair of air bearingssupporting the rotary shaft such that the rotary shaft is rotatablerelative to the housing, wherein a load on the first end portion islarger than a load on the second end portion, the pair of air bearingsincludes a first air bearing, and a second air bearing supporting therotary shaft at a position closer to the second end portion of therotary shaft than the first air bearing is, and a load carrying capacityof the first air bearing is larger than a load carrying capacity of thesecond bearing.
 2. The electric compressor according to claim 1, whereina length of the first air bearing is greater than a length of the secondair bearing in the axial direction so that the load carrying capacity ofthe first air bearing is larger than the load carrying capacity of thesecond air bearing.
 3. The electric compressor according to claim 1,wherein the first air bearing and the second air bearing have differentshapes so that the load carrying capacity of the first air bearing islarger than the load carrying capacity of the second air bearing.